Among the combustion means allowing CO2 capture, oxycombustion units afford the advantage of producing combustion fumes free of nitrogen from the combustion air since combustion is achieved from pure oxygen. Such a method is for example described in patent WO-2007/039,687 A. This oxygen is produced by an air separation unit (ASU). One drawback of this combustion mode and of ASUs in particular is their high energy consumption and their high investment cost that significantly increases the overall capture cost.
One solution thus consists in using a chemical looping combustion method. Chemical looping combustion involves a high potential in terms of energy efficiency and cost reduction. This method avoids the energy penalty linked with the separation of oxygen and air. It is based on the oxygen transfer capacity of some materials such as metallic oxides. An air reactor is used to oxidize the oxygen carriers prepared in form of fine particles that are transferred to a fuel reactor where they are reduced by combustion of the fuel. This method is generally carried out on a pilot scale in form of two fluidized beds exchanging solid streams: the air reactor being then a fast fluidization type reactor at the top of which the oxygen-depleted air stream and the particles are separated by a cyclone, the particles moving down through gravity in the fuel reactor consisting of a dense fluidized bed, where an overflow achieves reinjection of the solids at the bottom of the riser, while the combustion gases (essentially CO2 and H2O) are discharged through the overhead of this dense fluidized bed. Patent FR-2,850,156 notably describes the principle of chemical looping combustion, in a method dedicated to coal combustion.
In the case of solid fuels, unburnt residues remain at the reduction reactor outlet. They are carried along with the oxygen carrier into the air reactor where they are burned, which however produces CO2 mixed nitrogen, which affects the capture rate of the unit. To avoid this, it is necessary to have a specific equipment for separation between particles of different nature but of comparable size, hence the complexity of the system, in particular in the case of large-scale industrial extrapolation.
Tests have been carried out for chemical looping integration in hydrocarbon conversion plants.
For example, document WO-2007/082,089 A2 describes a three-stage method highlighting the use of metallic oxides recirculation for hydrogen production. In a first reactor, total combustion of the fuel allows to produce CO2, H2O. Hydrogen production is performed by reoxidizing the metallic oxide by means of steam. This method requires high steam flow rates, and it is therefore necessary to heat and to evaporate a large amount of water prior to feeding it into the oxidation reactor, which leads to a limiting energy balance.
Hydrogen production can also be achieved through gasification: patent application WO-2008/036,902 A2 describes for example a hydrocarbon gasification method that is implemented in a conventional layout with two reaction zones.
However, a problem that faces the person skilled in the art wanting to produce synthesis gas (therefore hydrogen) by gasification is the kinetics of the reactions that take place in the gasification reactor, as well as the high reaction temperatures in the gasification reactor. The residence time required for the reactants is thus long. This directly affects the size of the plants and, more specifically, the size of the reactors involved, which leads to high investment costs.
Some oxygen carriers have the capacity to spontaneously release part of their oxygen in an oxygen-poor medium. Thus, we have discovered that the presence of an oxygen production reactor within a chemical loop allows to gasify the fuel with an oxygen-enriched mixture while avoiding direct solid-solid fuel contact. It is thus possible to do without solid-solid separation equipments. This particular configuration furthermore affords the advantage of improving the energy balance of the gasification stage, very endothermic in the absence of oxygen, and of accelerating the reactions since the reactions that occur are reactions between a solid and a gas (and no longer between a solid and a solid). The method according to the invention is particularly advantageous for gasification of heavy feeds.